The collision occurred on the 138 Freeway and 300th St. West, east of Gorman at about 2:15 p.m. Tuesday.
The fire crew truck was traveling eastbound on the 138 when a white Subaru Forester traveling westbound crossed the center line and crashed into the truck.
The fire department truck reportedly rolled several times before coming to a rest, according to investigators. The fire-crew transport truck was overturned at the scene.
One person in the transport truck was killed and the driver of the passenger vehicle, its sole occupant, was killed.
Crew members in the truck were thrown from the vehicle. One of the crew members, an inmate, was pronounced dead at the scene. Other passengers in the truck were transported by air or ground to area hospitals. Four people were listed in critical condition and eight more were wounded, according to the L.A. County Fire Dept.
The Forester's driver was pronounced dead at the scene. He was described as an elderly man between 70 and 80 years old. The car had Nevada license plates.
The passengers on the fire truck are part of a state prison firefighting crew. The foreman, a fire captain, was driving the truck. The inmates were low-security and were not handcuffed. The crew was headed back to their camp near the crash site in San Francisquito Canyon in Saugus after completing a project.
Authorities did not release the identity of the deceased elderly man.
The inmate was identified as Fernando Sanchez, 25, from San Diego County.
The investigation was expected to continue into Tuesday night.
The 138 was closed in both directions while the collision was investigated.
The Associated Press contributed to this report.